GCSE Geology Igneous Rocks Introduction Click anywhere on

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GCSE Geology Igneous Rocks Introduction Click anywhere on the screen to move on. GCSE

GCSE Geology Igneous Rocks Introduction Click anywhere on the screen to move on. GCSE Geology

Igneous Rocks GCSE Geology

Igneous Rocks GCSE Geology

Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling,

Definition of Igneous Derived from the latin ‘ignis’ meaning fire Formed by the cooling, crystallization and solidification of molten lava or magma Comprise an interlocking mosaic of crystals GCSE Geology

Extrusive Igneous Rocks Molten rock reaching the Earth’s surface via volcanoes (lava) is termed

Extrusive Igneous Rocks Molten rock reaching the Earth’s surface via volcanoes (lava) is termed extrusive GCSE Geology

Intrusive Igneous rocks Molten rock (magma) that solidifies at depth within the lithosphere is

Intrusive Igneous rocks Molten rock (magma) that solidifies at depth within the lithosphere is intrusive Intrusive rocks may eventually be exposed at the Earth’s surface following a long period of uplift and erosion GCSE Geology

Crystal Size and Cooling Rates Crystal size is determined by the rate of cooling

Crystal Size and Cooling Rates Crystal size is determined by the rate of cooling of the magma or lava Instantaneous cooling of lava erupted under water as pillow lavas results in a glassy texture, with no crystals GCSE Geology

Crystal Size and Cooling Rates Rapid cooling in at the earth’s surface over a

Crystal Size and Cooling Rates Rapid cooling in at the earth’s surface over a few months results in crystals of <1. 0 mm in diameter forming lava flows (volcanic) GCSE Geology

Crystal Size and Cooling Rates Minor intrusions-dykes and sills cool slower over thousands of

Crystal Size and Cooling Rates Minor intrusions-dykes and sills cool slower over thousands of years to form crystals 1. 0 mm-3. 0 mm in diameter GCSE Geology

Crystal Size and Cooling Rates Slow cooling in magma chambers deep underground over millions

Crystal Size and Cooling Rates Slow cooling in magma chambers deep underground over millions of years results in larger crystals >3. 0 mm in diameter (plutonic) GCSE Geology

Phenocrysts Large well formed crystals in an igneous rock In Shap Granite the pinkish

Phenocrysts Large well formed crystals in an igneous rock In Shap Granite the pinkish coloured orthoclase feldspar phenocrysts are up to 3 cm in length GCSE Geology

Groundmass The remainder of the igneous rock made up of smaller crystals In the

Groundmass The remainder of the igneous rock made up of smaller crystals In the case of Shap Granite, the groundmass is mainly crystals of biotite mica and quartz GCSE Geology

Phenocrysts and Groundmass Orthoclase phenocrysts up to 6 cm in length Phenocrysts are rectangular

Phenocrysts and Groundmass Orthoclase phenocrysts up to 6 cm in length Phenocrysts are rectangular Implies 2 stage cooling history 1 cm Finer groundmass 0. 51. 0 mm in diameter GCSE Geology

Equicrystalline Texture All the crystals in the rock are roughly the same size Produced

Equicrystalline Texture All the crystals in the rock are roughly the same size Produced by a steady or constant cooling rate GCSE Geology

Equicrystalline Texture 2 cm Microgranite – even cooling, all crystals 1. 0 – 1.

Equicrystalline Texture 2 cm Microgranite – even cooling, all crystals 1. 0 – 1. 5 mm GCSE Geology

Porphyritic Texture Large crystals (phenocrysts) set in a finer grained groundmass Formed by two-stage

Porphyritic Texture Large crystals (phenocrysts) set in a finer grained groundmass Formed by two-stage cooling GCSE Geology

Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5 cm long Long axes of phenocrysts

Porphyritic Texture-Giant Feldspar Porphyry Phenocrysts up to 5 cm long Long axes of phenocrysts aligned parallel implies flow of magma Groundmass 1. 0 -1. 5 mm GCSE Geology

Vesicular Texture Spherical or ellipsoidal cavities found in lavas Vesicles represent gas bubbles trapped

Vesicular Texture Spherical or ellipsoidal cavities found in lavas Vesicles represent gas bubbles trapped in lava or magma GCSE Geology

Vesicular Texture Vesicles represent trapped gas bubbles in this lava flow Vesicles range from

Vesicular Texture Vesicles represent trapped gas bubbles in this lava flow Vesicles range from 2 mm to 1. 5 cm in diameter Vesicles are stretched and curved indicating flow of the lava Car key for scale GCSE Geology

Glassy Texture e. g. in Obsidian No crystals visible due to very rapid cooling

Glassy Texture e. g. in Obsidian No crystals visible due to very rapid cooling 1 cm Shows Conchoidal Fracture like glass GCSE Geology

Mineral Content Igneous rocks are classified chemically as Silicic or Mafic according to the

Mineral Content Igneous rocks are classified chemically as Silicic or Mafic according to the main constituent minerals present GCSE Geology

Silicic Igneous Rocks Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite Mica and Muscovite Mica. Rich

Silicic Igneous Rocks Quartz, Orthoclase Feldspar, Plagioclase Feldspar, Biotite Mica and Muscovite Mica. Rich in silica >66% GCSE Geology

Mafic Igneous Rocks Plagioclase Feldspar, Augite and Olivine Contain less silica 45 – 52%

Mafic Igneous Rocks Plagioclase Feldspar, Augite and Olivine Contain less silica 45 – 52% GCSE Geology

The Classification of Igneous Rocks % Silica Content Over 66% 52% to 66% Chemical

The Classification of Igneous Rocks % Silica Content Over 66% 52% to 66% Chemical Group Silicic % Light Minerals 60% or over Intermediate 40%-70% 30%-60% 45% to 52% Below 45% % Dark Minerals 40% or below Mafic Ultramafic GCSE Geology 70%- 90% 10%-30% 90% or over 10% or below Rock Names Rhyolite/Obsidian/Pumice (Fine) Micro-granite (Medium) Granite (Coarse) Andesite (Fine) Micro-diorite (Medium) Diorite (Coarse) Basalt (Fine) Dolerite (Medium) Gabbro (Coarse) - Peridotite (Coarse)

Cornish Granite Glassy, colourless quartz 1 cm All crystals over 3. 0 mm in

Cornish Granite Glassy, colourless quartz 1 cm All crystals over 3. 0 mm in diameter Black biotite mica with pearly lustre White/creamy plagioclase feldspar GCSE Geology

Shap Granite Porphyritic texture, large phenocrysts and finer groundmass Finer groundmass of quartz and

Shap Granite Porphyritic texture, large phenocrysts and finer groundmass Finer groundmass of quartz and biotite mica 2 -3 mm in diameter Feldspar phenocrysts are well shaped 1 cm Pink coloured orthoclase feldspar phenocrysts up to 3 cm long GCSE Geology

Micro-Granite Formed within the crust in a sill or dyke Mineralogy: quartz, feldspar and

Micro-Granite Formed within the crust in a sill or dyke Mineralogy: quartz, feldspar and mica Equicrystalline texture, all crystals 1. 0 – 1. 5 mm in diameter Formed by an even cooling rate over thousands of years GCSE Geology 2 cm

Gabbro Equicrystalline texture, all crystals roughly similar in size Greenish-black augite Formed deep underground

Gabbro Equicrystalline texture, all crystals roughly similar in size Greenish-black augite Formed deep underground by very slow cooling over millions of years 2 cm Coarse grained, crystals over 3. 0 mm in diameter, suggesting slow cooling GCSE Geology Grey/creamy plagioclase feldspar

Porphyritic Dolerite Medium crystal size mainly 1. 0 – 2. 0 mm Mineralogy: plagioclase

Porphyritic Dolerite Medium crystal size mainly 1. 0 – 2. 0 mm Mineralogy: plagioclase feldspar, augite and olivine Groundmass constitutes over 75% of the rock Subhedral phenocrysts of plagioclase feldspar up to 3 mm in diameter Two-stage cooling, finally forming an intrusive dyke or sill GCSE Geology 1 cm

Basalt 1 cm Chilled margin, very fine grained Formed by rapid almost glassy cooling

Basalt 1 cm Chilled margin, very fine grained Formed by rapid almost glassy cooling at the earth’s surface over a few weeks or months Mineralogy: plagioclase feldspar, augite and olivine Crystal size is fine, well under 1. 0 mm, (volcanic) GCSE Geology

Peridotite 1 cm Coarse grained >3. 0 mm Ultramafic Low silica content Predominantly olivine

Peridotite 1 cm Coarse grained >3. 0 mm Ultramafic Low silica content Predominantly olivine Small amount of augite Tiny amount of feldspar Mantle rock GCSE Geology